1. Field of the Invention
The present invention relates to a charging control system and a charging control device for controlling the charging of a secondary battery from a solar battery and reducing the loss of power from the secondary battery when the charging path is interrupted.
2. Description of the Related Art
A known type of charging control system, disclosed by Yamada et al. in Japanese Patent Application Publication No. 9-261861, controls the charging of a secondary battery from a solar battery by means of a switching element that prevents the secondary battery from discharging through the solar battery when not being charged by the solar battery.
The circuit configuration of this charging control system is shown very schematically in FIG. 1. The main elements are a solar battery 1, a secondary battery 2, and a charging path 3 interconnecting the solar battery 1 and secondary battery 2. For clarity, reference characters 3H and 3L are used to distinguish the side of the charging path 3 interconnecting the anodes of the solar battery 1 and secondary battery 2 from the side of the charging path 3 interconnecting the cathodes of the solar battery 1 and secondary battery 2. The system also includes a current path 4 interconnecting the high side charging path 3H and low side charging path 3L through resistors R1 and R2, a current path 6 interconnecting the high side charging path 3H and low side charging path 3L through resistors R3 and R4, a comparator 7 for comparing the voltage of the solar battery 1 as divided by resistors R1 and R2 with the voltage of the solar battery 1 as divided by resistors R3 and R4, and a p-channel metal-oxide-semiconductor (PMOS) transistor 12 inserted in high side charging path 3H between current path 4 and current path 6 to interrupt the charging path 3 responsive to the output of the comparator 7. The secondary battery 2, part of the charging path 3, and current path 4 form a closed circuit.
This conventional system operates as follows. The comparator 7 compares the voltage of the secondary battery 2 as divided by resistors R3 and R4 on current path 4 with the voltage of the solar battery 1 as divided by resistors R1 and R2 on current path 6. If the difference between the voltage of the solar battery 1 and voltage of the secondary battery 2 exceeds a threshold value, the comparator 7 outputs a low-level signal to turn on the PMOS transistor 12, thereby charging the secondary battery 2 from the solar battery 1; if the difference is equal to or less than the threshold value, the comparator 7 outputs a high-level signal to turn off the PMOS transistor 12, thereby interrupting the charging of the secondary battery 2 from the solar battery 1.
A problem in this conventional charging control system is that even when the switching element is turned off to interrupt the charging of the secondary battery from the solar battery, current continues to flow from the anode of the secondary battery 2 to the cathode of the secondary battery 2 through current path 4, thereby discharging the secondary battery 2 and wasting some of the power stored in the secondary battery 2.